To add a customized message to your invoices during the booking process (either manually or online), just follow these simple steps.
Click the "Add a Custom Message..." link under the Proposal settings column.
On the next screen you can type in any customized message that you would like to appear on the invoice. You can also save this message as a default so that it appears on your invoices every time you go through the booking process.
When your client views their invoice they will see your customized message. The exact location of this message on the invoice is determined by which theme you are using for your brand (under Settings > Logos & Colors). Basically, though, the message will appear under the itemized contents of the package/order.
You can always change or remove the message by Rebuilding the Order.